It is well-known that laser glasses must be able to withstand high internal temperatures created by flash lamps and the like without experiencing significant distortion, cracking or any significant change in optical properties. Commonly referred to as thermal shock resistance, optical elements such as laser rods, slabs, discs and fibers must be capable of enduring high-frequency optical pumping without catastrophic failure. In operation, a laser glass element acquires heat from the pumping light source. In order to dissipate this heat, laser devices are typically liquid-cooled which helps prevent thermal rupture of the laser glass element. However, laser glass rods and the like still develop large internal temperature gradients, causing thermally-induced stress across the element. At high powers and high repetition rates, conventional laser glasses may fracture as the result of large thermal gradients produced by internal optical pumping.
It is also known that thermal distortion of laser glass elements produces a variance in refractive index which may cause optical distortion of the laser beam. Hence, it is important to provide laser glasses which have a small change in refractive index over broad temperature ranges. In some instances it is even desirable to provide laser glass elements having a negative change in refractive index to partially compensate for optical distortions produced by a positive coefficient of thermal expansion. These thermal effects are also known to diminish the energy efficiency of active laser components. Therefore, it would be desirable to provide glass compositions which could be formed into optical components having good mechanical strength and high thermal shock resistance for use as laser elements, filters and the like.
A number of methods for strengthening silicate-based glasses have been proposed, including ion-exchange or "stuffing" of the surface of a silicate glass article with large ions in exchange for small ions below the softening temperature of the glass. For example, one such method is disclosed in "Stresses in Glass Produced By Non-Uniform Exchange of Monovalent Ions," J. Am. Ceram. Soc. 45 [2] 59-68 (1962) wherein a chemical method of silicate glass strengthening is disclosed which involves low-temperature ion exchange. Other investigators, notably Nordberg et al., "Strengthening By Ion Exchange," J. Am. Ceram. Soc. 47 [5] 215-219 (1964), have described low-temperature ion exchange strengthening of silicate glasses. Ion exchange treatment of lithia silicate laser glasses is also discussed in U.S. Pat. No. 3,687,799, entitled "Glass Lasers of Increased Heat Dissipation Capability Made By Ion Exchange Treatment of Laser Glass." However, very little research has been conducted on the strengthening of non-silicate glasses.
In U.S. Pat. Nos. 4,075,120 and 4,248,732, both of which are assigned to the assignee of the present invention and which are incorporated herein by reference, novel phosphate glass compositions particularly suitable for forming laser glass elements are disclosed. As will be shown, the present invention provides phosphate glass compositions which are unique in their ability to be ion-exchanged. That is, the phosphate glass formulations of the present invention produce unexpected superior results over the prior art.
In U.S. patent application "Ion-Exchangeable Germanate Glass Compositions and Strengthened Germanate Glass Articles," filed Oct. 27, 1987, and assigned to the assignee of the present invention, novel germanate glass compositions and articles formed of these germanate glasses are disclosed which exhibit good mechanical strength. However, although phosphate glasses have unique properties which make them superior to silicate, borate and most other glasses for certain applications, to Applicant's knowledge, no one has successfully ion-exchanged a phosphate glass composition to produce optical quality thermal shock resistant glass articles as provided by the present invention. Therefore, there exists a long-felt need for phosphate glass compositions from which thermal shock resistant articles can be made and for optical quality glass articles which exhibit good mechanical strength and which have a high degree of thermal shock resistance. The present invention provides ion-exchangeable phosphate glass compositions and optical quality strengthened phosphate glass articles which satisfy this need.